1. Field of the Invention
The present invention relates to a message transfer method and apparatus in an optical transmission system and, more particularly, to a message transfer method that can increase the maximum number of nodes that can be connected on one ring in a BLSR (Bi-directional Line Switched Ring) network configuration.
2. Description of the Related Art
A BLSR switching control system is implemented conforming to the North American SONET standard GR-1230-CORE. A major feature of the BLSR control system is that line utilization efficiency increases because traffic can be restored in the event of a failure by using a protection line going in the opposite direction along the ring, allowing the protection line for the same channel to be used for another line. The recent trend for the network configuration of optical transmission systems is toward further increases in line capacity and network size backed by technological innovations; with this trend, coupled with the above reason, demand for the BLSR control system is increasing rapidly, and the need for its application to larger ring networks is growing.
In the BLSR control system, switching control is effected by transferring between nodes the K1/K2 bytes contained in the line overhead of the main signal. The K1/K2 byte format defined in GR-1230 is shown in FIG. 1.
When a line failure or the like occurs in a network that uses a BLSR topology such as shown in FIG. 2, the nodes adjacent to each other across the failed span send K1/K2 bytes carrying a switching request, in response to which the adjacent nodes perform a switching operation (and each intermediate node performs a pass-through operation) to restore the traffic from the failure. Furthermore, in order to prevent erroneous connections, etc. from occurring due to switching operations when multiple failures occur on the ring, a unique ID is assigned to each node on the ring, and all the nodes on the ring are made to recognize a ring map defining the ordering of the IDs, to ensure reliable switching operations.
Each node must identify from the received K1/K2 bytes whether the switching request is addressed to itself or not, and must control the switching operation (or pass-through operation) by distinctly recognizing from where and over which path the switching request has arrived (see FIG. 2) by checking against the ring map. For these purposes, “DESTINATION NODE ID”, “SOURCE NODE ID”, and “PATH”, as well as “SWITCHING REQUEST” and “SWITCHING STATUS”, are set in the K1/K2 bytes, as shown in FIG. 1.
As an example of the APS (Automatic Protection Switching) protocol using the K bytes, an outline of a ring switching procedure will be described by dealing with the case in which a failure has occurred in the line from #3 to #2 in the network of FIG. 2. Upon detection of the failure, node #2 on the receiving side recognizes from the ring map that the adjacent node is #3, and sends a switching request to the node #3 over a long path (and also over a short path). Since “DESTINATION NODE ID” indicates #3, each intermediate node recognizes that the request is not addressed to itself, and passes the K bytes onto the next node and so on, until the K bytes reach the node #3. The node #3 recognizes that the request is addressed to itself, and knows that a failure has occurred on its outgoing line. The node #3 then returns a response to the switching request to the source node #2. When the switching request response is confirmed between the nodes #2 and #3, the two nodes perform a ring switch.
That is, for BLSR switching control, it is essential that each node on the ring has a unique ID and be able to uniquely identify a “SOURCE NODE” and a “DESTINATION NODE” from the K1/K2 bytes being passed around the ring.
In the previous K1/K2 byte format shown in FIG. 1, “DESTINATION NODE ID” and “SOURCE NODE ID” are each assigned only four bits, with which only 0 to 15 can be defined; therefore, the limitation is that a maximum of 16 nodes can be connected on one ring. GR-1230 defines a method called “ring interconnection” for interconnecting rings to enable the construction of a network ring consisting of more than 16 nodes, but compared with a BLSR constructed with a single ring, this obviously increases the complexity of control as well as the initial equipment cost for the interconnection. Furthermore, the ring interconnection means simply interconnecting a plurality of rings, and the network as a whole does not provide the redundant configuration, unique to the BLSR, that cycles around the ring.
Though it is strongly desired to support a wide area with one ring while making use of the advantage of efficient line utilization of the BLSR topology, the reality is that, with the APS protocol based on the current K/K2 byte format, the limitation of 16 nodes per ring is an unavoidable bottleneck.
Since the K1/K2 bytes are carried for transmission in the K1/K2 byte area defined within the line overhead of the main signal, expanding the K1/K2 byte area itself is extremely difficult as it would require major modifications to the hardware. Furthermore, redefining the “SWITCHING REQUEST” and “SWITCHING STATUS” fields is not easy, because then the APS protocol would have to be redefined.
If more than 16 nodes could be identified by making intelligent use of the “DESTINATION NODE ID”, “SOURCE NODE ID”, and “PATH” fields in the existing K1/K2 byte format while keeping the “SWITCHING REQUEST” and “SWITCHING STATUS” fields unaltered, a BLSR network with more than 16 nodes on one ring could be constructed without having to expand the K1/K2 byte field and without redefining the BLSR switching control protocol, and all the previous BLSR switching functions could be accomplished while keeping changes from the GR-1230 standard to a minimum.